1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device, and more particularly, to a vertical nitride semiconductor light emitting device improved in light extraction efficiency.
2. Description of the Related Art
Recently, a nitride semiconductor light emitting device has been spotlighted in related technical fields as a high-power optical device capable of generating light of wide wavelength band including light of short wavelength such as blue or green light. The nitride semiconductor device is made of a semiconductor single crystal having a composition expressed by AlxInyGa(1-x-y)N, where 0≦x≦1, 0≦y≦1 and 0≦x+y≦1.
In general, light efficiency of the nitride semiconductor light emitting device is determined by internal quantum efficiency and light extraction efficiency (also called external quantum efficiency). Especially, light extraction efficiency is decided by an optical parameter of the light emitting device, i.e., refractivity of each structure and/or flatness of an interface.
In terms of light extraction efficiency, the nitride semiconductor device bears fundamental limitations. That is, a semiconductor layer of the semiconductor light emitting device has higher refractivity than an external atmosphere or substrate. This reduces a critical angle determining an incidence angle range of light that can be extracted, consequently causing a great portion of light generated from the active layer to suffer total internal reflection and to propagate in an undesired direction. Alternatively, a majority of light is lost during total internal reflection, thus lowering light extraction efficiency.
To overcome problems in light extraction efficiency as described above, Japanese Patent Application Publication No. 2002-368263 (published on Dec. 20, 2002, assigned to Toyota Gosei KK) teaches a flip-chip nitride light emitting device having a roughened underside as shown in FIG. 1a. 
Referring to FIG. 1a, the nitride semiconductor light emitting device 10 according to the aforesaid document includes a sapphire substrate 11, a first conductivity-type nitride semiconductor layer 14 formed on the sapphire substrate 11, an active layer 15 formed on the first conductivity-type nitride semiconductor layer 14, and a second conductivity-type nitride semiconductor layer 16 formed on the active layer 15. Also, a buffer layer 12 is formed on the sapphire substrate to enhance crystalinity of the nitride semiconductor layer. The nitride semiconductor light emitting device 10 includes first and second electrodes 19a and 19b connected to the first conductivity-type nitride semiconductor layer 14 and the second conductivity-type nitride semiconductor layer 16, respectively. Further, an underside of the sapphire substrate 11 is roughened via etching to form a light scattering surface.
To manufacture a flip-chip nitride semiconductor light emitting device 20 as in FIG. 1b, the nitride semiconductor light emitting device 10 shown in FIG. 1a is mounted onto a package substrate 21 having first and second conductive lines 22a and 22b. In addition, the electrodes 19a and 19b are connected to the first and second conductive lines 22a and 22b by connecting means S such as soldering. At this time, the underside 11a of the sapphire substrate 11 serves as a light scattering surface where light exits. Light generated from the active layer 15 directly heads toward the light exiting surface 11a (see reference sign a) or is reflected from a bottom surface toward the light exiting surface 11a (see reference sign b). Then light reaching the light exiting surface 11a is scattered on the roughened underside of the sapphire substrate or exits effectively due to a big critical angle resulting from the pattern with microstructural features.
However, typically, to grow a nitride, a sapphire substrate with high hardness is used. Therefore, it is not an easy process to form a roughened surface, i.e., a rough pattern with microstructural features on the sapphire substrate. Also, a difficult process control renders it hard to form a desired rough pattern.
Meanwhile, the aforesaid conventional nitride light emitting device is limited to a planar structure using an insulating sapphire substrate. The conventional technology is hardly applicable to a vertical nitride light emitting device which has recently drawn considerable attention. That is because the vertical nitride light emitting device has an electrode formed on a light extraction surface where a roughened light scattering surface will be formed.
Also, in the vertical nitride light emitting device, a rough pattern needs to be formed directly on a nitride layer or a conductive substrate such as GaN according to the aforesaid conventional technology. However, high hardness similar to that of the sapphire substrate renders it hard to form the rough pattern via typical wet-etching, thus requiring dry-etching such as ICP. However, disadvantageously, this complicates a process and does not ensure a desired rough pattern.